Not to sound like a ****, because i'm no pro myself. But if you're asking this question you'd probably wanna keep it simple, go for the americana or the kimura.
If on the other hand, i misunderstood you, and he was on top of you, you're primary concern needs to be getting free without being submitted.
Or as guys sharper than me say "Position then submission"

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"Remember when you are resting, someone somewhere is training, and when you meet him, he will win."

I know that this may sound rude, but you can submit a guy from anywhere. From every position in BJJ there is an escape and a submission, that's just the way it is.

Really, if you are in a mount I suggest a straight choke, put your forearm on his throat and just lean your weight forward so that he starts to choke. In gi grappling I actually grab the inside of his collar, which makes it alot harder to get my hand and my weight off of him.

The other really popular ones from this seem to be the keylock, which can be kind of complicated for a beginner, or you can swing around one of your opponent's arms and into a straight armbar.

While getting out may be the first concern for a beginner, it isn't as suprising. I do agree that you should use the up if you're trying to just get out. If you grab the arm then you can role it into an armdrag and submit your opponent from there.

The other locks include an elbow role that can turn your opponent away from you. This can lead into one of a half dozen armlocks.

The up escape sets up some leg submissions and give you their back, at least for a second.

I would agree with Mr. Yeti here. Your first concern would be to get out of that situation. The only bright side of this is that you do have options...

Option #1: The "Upa" or Bridge Escape
This is the basic bridge escape from the bottom. This is also what all your other options will be built off of. This is your first go to move; as if it is thwarted you should be able to create a little wiggle room to go to the other options that will be listed below. Here's the Upa in steps:
1) Choose a side that you will bridge to. This will determine what direction you will bridge to, and what side of your opponent you will try to control. For illustration purposes, we'll say you’re going to bridge to your right (but this can be done-and should be drilled-off of your left side also).
2) Take control of your opponent’s left arm by securing your right hand on his left triceps, and your left hand on his left wrist. You can do this by also securing both his arms by grabbing at both his wrists, but the 2-on-1 control is a lot better.
3) Move your right foot up and outside your opponent’s left foot, blocking his left leg high and tight to yours. While you do this, bring your left foot up to your buttocks (you will be using your left leg as the primary leg to push off of, and if it is stretched away from your body and flat on floor it will have no power). When you are at this point, you will have isolated any movement on his left side which allows the sweep to happen.
4) Drag his left arm across and down your body. When you do this, gravity will work in you favor and bring his left side down toward the floor toward his shoulder.
5) While you are dragging your opponent’s arm across your body, bridge up hard using your left leg. You want to execute a full, explosive bridge from your toes all the way up to the top of your head. Also you do not want to just bridge up, neither do you want to just bridge up and turn to your right. What you want to do is bridge up and over your own right shoulder.
6) This should get your opponent rolling off of you over his own left shoulder. Continue over with the momentum until you end up on top in your opponent’s guard. Congratulations—you are now out of your opponent’s mount!

A few things to keep in mind:

First and foremost: DO NOT PANIC. Yeah, being caught in the mount sucks. However, if you panic and start flailing and twisting and turning, not only are you burning yourself out, but you are also making yourself bait for all sorts of nasty submission holds. So keep calm and thinking. Remember that you have options to get out.

Do not just bridge up and directly to your right. If you do that, your opponent can stop the Upa by wiggling his left leg free and posting it out. I will post later on what to do if he does that, but I digress… You want to bridge out at a 45 degree angle over your right shoulder. Think of it this way: If you were laying down on a clock face with your head at 12 and your feet at 6, you will be bridging more in the direction of 10 or 11 o’clock. This way if your opponent still posts out their leg, you still have the chance of pulling them over because they can’t use their arm to stop the roll and the direction of the roll is actually going away from their leg.

When you complete the roll, post your arms out and establish a base. If you don’t, your opponent can continue rolling with the momentum you’ve created and end up right on top of you. This time, not only are you mounted, but you’ve just spent a lot of energy trying to get out, only to fail.

I'll address the Elbow escape (from which all your sobmission options will start) a little later. But I think this will help.

Wow thanks Onganju, extremely helpful advice, there are actually a lot of possibilities
Also thank you to Yeti and Ironman, great advice.

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Fun trivia: In danish "kamp" means "match" or "fight". So Martin Kampmann really lives up to his name.

Really, if you are in a mount I suggest a straight choke, put your forearm on his throat and just lean your weight forward so that he starts to choke. In gi grappling I actually grab the inside of his collar, which makes it alot harder to get my hand and my weight off of him.

Ive found that unless you overpower the other guy this doesnt seem too effective because they can push the arm out a little bit, then they have one of your arms and can go for the escape. from mount i personally prefer the straight armbar just because its hard to get out of once you have it sunk. keylock is good too. just my personal experience, not meant to be a discredit to the advice.

Ive found that unless you overpower the other guy this doesnt seem too effective because they can push the arm out a little bit, then they have one of your arms and can go for the escape. from mount i personally prefer the straight armbar just because its hard to get out of once you have it sunk. keylock is good too. just my personal experience, not meant to be a discredit to the advice.

This might sound straight up rude, but if you need to over power him then you're not doing it right. Ground yourself and apply pressure straight down through his throat. It can actually be pretty dangerous, so alot of people don't apply full pressure, but if you lift up and relieve pressure then you can be over powered.

I have never trained with anyone who can fight back while I am applying what is essentially my bodyweight to their trachea. While this sounds dangerous, all they need to do is tap and all submissions have a certain level of danger.

When I was first introduced to aikido the big thing I learned abot applying pins (which I have applied in this particular choke) is to put all of my body weight onto the pinning limbs, like a handstand. Apply this technique gradually, because they almost always tap before you get your full bodyweight on top of them.

my point was that for me this is difficult because i dont have very much body weight, and i have a tendancy to roll with stronger guys because im tall, so this is a little less effective when i do it. sorry if i wasnt clear

I know what you mean, I'm a little guy too. What I'm saying is that there are ways to apply chokes that can, for the most part, negate the strength of a bigger fighter. If your opponents grabs your arm to prevent the choke then turn it into a straight armbar or an Americana.

I understand that for beginners this might not be something that's easy to do, but practice makes perfect. I learned these technique combos by f*cking them up hundreds of times, now I can, for the most part, get them right.